Method of preventing rack plating in continuous plating cycle for nonconductive articles

ABSTRACT

A METHOD IS DISCLOSED FOR TREATING PLASTIC SHEATHED METAL JIGS OR RACKS CONVENTIONALLY USED IN SUPPORTING AND TRANSPORTING NONCONDUCTIVE ARTICLES THROUGH CHEMICAL AND ELECTROPLATING SOLUTIONS IN THE COURSE OF A CONTINUOUS PLATING CYCLE FOR METALLIZING THE ARTICESS, WHEREBY TO PREVENT OR REDUCE DEPOSIT OF METAL ON THE RACKS THEMSELVES. O THIS END, THE RACKS ARE TREATED TO CAUSE ADSORPTION OF HEXAVALENT CHROMIUM IONS AT THE SURFACE OF THEIR PLASTIC SHEATHED PROTIONS.

United States Patent 3,592,744 METHOD OF PREVENTING RACK PLATING INCONTINUOUS PLATING CYCLE FOR NON- CONDUCTIVE ARTICLES John J. Grunwald,New Haven, Eugene D. DOttavio,

Thomaston, and Frank L. Durso, Cheshire, Conn, assignors to MacDermidIncorporated, Waterbury, Conn. N0 Drawing. Filed Dec 2, 1968, Ser. No.780,567 Int. Cl. C2311 /60; B44d 1/092; B231) 27/00 US. Cl. 204-20 5Claims ABSTRACT OF THE DISCLOSURE A method is disclosed for treatingplastic sheathed metal jigs or racks conventionally used in supportingand trans porting nonconductive articles through chemical andelectroplating solutions in the course of a continuous plating cycle formetallizing the articles, whereby to prevent or reduce deposit of metalon the racks themselves. To this end, the racks are treated to causeadsorption of hexavalent chromium ions at the surface of their plasticsheathed portions.

The art of metallizing plastics both for decorative and functionalapplications is undergoing rapid growth. The general method employedconsists of chemically depositing on the plastic object a preliminary orinitial conductive metallic coating, Whereafter it can be electroplatedby known electro-chemical techniques as if the objects were made ofmetal. The successful application of the initial conductive metalliclayer is of crucial importance for the subsequent successfulelectroplating of one or more layers of the same or different metal tobuild up a total metallic deposit of desired thickness and finishcharacteristics. Copper, nickel, chromium and sometimes cobalt are themetals most commonly applied to commercial articles in the automotiveappliance, plumbing and related industries today.

There are several commercial methods for metallizing plastics. Allconsist of immersing the object to be plated first in a number ofaqueous or nonaqueous systems comprising an electroless or chemicalplating cycle, each of such systems contributing to a specificalteration of the polymer surface of the object to be plated to cause itto accept a thin adherent initial metallic layer on which heavierdepositscan subsequently be deposited electrochemically, i.e. byconventional electroplating procedures. In the foregoing operations, thearticles to be plated are racked, that is suspended from plating jigs orracks adapted to support and transport the articles during their passagefrom one plating treatment solution to another as well as duringimmersion in each of such plating solutions. It is also a function ofsuch racks to make electrical contact between the objects and thecathode bar of the electroplating tank or tanks while the articles areundergoing treatment in that portion of the overall cycle of operations.However, it is a practical necessity to provide some means of preventingdeposit of metal plate on the conductive racks themselves, since this isnot only wasteful of plating metal and plating current but is alsodisruptive of proper, uniform plating of the articles. The racks areaccordingly coated with a sheath of polymeric electrically insulatingmaterial, except for the terminal clips and wires where electricalcontact must be established with the parts to be electroplated.Difiiculty arises, however, even though the racks are thus insulated, ifthey are used in supporting and transporting the articles through theinitial electro- 3,592,744 Patented July 13, 1971 less plating portionof the metallizing operation, since the plastic sheathing on the rackswill normally react in much the same way as the plastic articlesthemselves to receive and retain an initial metallic film which ofcourse will then accept additional metal plate in the subsequentelectroplating portion of the cycle. Where this occurs, the racks mustbe periodically stripped of deposited metal by mechanical or chemicalmeans. The process is time-consuming and damaging to the racks,resulting in accelerated obsolescence of them. It is the avoidance ofthis problem to which the present invention is primarily directed.

In order to afford a better understanding of the problem, a descriptionof a typical method used in industry for metallizing plastic substrateswill be helpful for purposes of reference. The operations in aconventional plastic article metallizing cycle comprise the stepsenclosed in solid lines in the flow sequence shown below, While thesteps enclosed in dotted lines represent alternatves in the procedure inaccordance with this invention:

. I E Step A i i Anti-plate treatment of i plating rack by immersion inaqueous solution of hexavalent chromium ions. I I

Step 1 Hang plastic articles to be plated on conveyor-carried platingracks and immerse articles and racks in organic solvent pretreatment 0ution.

S 2 Water rinse.

Step A l Same as Step A. i

Step 3 i Immerse racked articles in chromic-sulfuric acid etch solution.

Step 4 Immerse racked articles in water and/or chrome reducing rinse.

Step 5 Immerse racked articles in stannous chloridehydroehloric acidsensitizing solution.

Step 6 Water rinse.

Step 7 Immerse racked articles in palladium-ehloritie-hydrochloric acidactivating solution.

(Continued on following page) (Continued from preceding page) Step 8Water rinse.

Step 9 Immerse activated plastic articles while still on rack inchemical plating solution containing a reducible salt of the metal to beinitially deposited on the articles.

Step Water rinse.

Step 11, etc. Immerse chemically plated articles while still on sameracks in conventional electroplating solutions to electrodeposit one ormore layers of the same or different plating metals to provide the finaldesired total thickness and external finish.

Water rinse, remove finished articles from racks and return latter tostart of cycle.

Most of the compositions employed in the prior art cycle of operationsare well known in the industry, including the chromic-sulfuric acid etchof Step 3, and the sensitizing, activating and chemical platingsolutions of Steps 5, 7 and 9. See for example, US. Pat Nos. 2,430,-581; 2,702,253; 2,766,138; 2,871,142 and 3,075,855. Further details ofthe organic solvent pretreatment, Step 1, are disclosed in copendingU.S. appln. Ser. Nos. 654,901 now Pat. No. 3,486,361 and 717,006. Step 4involves the removal of excess hexavalent chromium from the parts,following the etch treatment in Step 3, and depending on the plasticbeing plated and particulars of the subsequent plating proceduresselected, may be simply a thorough water rinse or a chrome-kill solutionof phosphoric acid or a surfactant solution of the type disclosed incopending appln. Ser. No. 758,589. In some instances the separatesensitizing and activating operations of Steps 5 and 7 are replaced by acolloidal palladium-tin chloride activator solution of the typedisclosed in copending US.

appln. Ser. No. 654,307 (now Pat. No. 3,532,518, issued Oct. 6, 1970)and this is followed generally by acceleration in a fluoroboric acidsolution.

The foregoing sequence of steps has enjoyed considerable commercialsuccess and is used in many practical operations. However there arenumerous variations and deviations in one or more of the steps which insome cases provide improved operation depending on the nature of thepolymer from which the article to be plated is formed. For example,while immersion of an article in Step 3 of the foregoing procedure isessential for complete coverage and good metal-to-polymer adhesion inthe case of plastic substrates of ABS (acrylonitrile-butadiene-styrene),it may be deleterious for successful coverage of polyamide substratematerials, such as nylon, for example, and is generally omitted from theplating sequence for such materials. On the other hand, nylon andpolypropylene substrates generally require preplating treatment in anorganic solvent solution, Step 1, while such step is unnecessary in thetreatment of ABS substrates. Thus it requires specific adaption of Steps1 through 9 in the foregoing outline to accommodate the nature of agiven polymer undergoing metallizing. Once the substrate has beenmetallizcd in the chemical plating portion of the procedure, allsubstrates behave identically thereafter and can be electroplated underessentially the same conditions.

It will be apparent from the foregoing that if the same plating racksare employed in transporting the articles through the various steps ofthe chemical plating portion of the cycle, i.e. Steps 19, as are usedfor continuing the treatment of the articles in the electroplatingportion of the cycle, i.e. Step 11 etc., the plastic coating orsheathing of the plating racks may also become metallized, making theentire rack electrically conductive and causing it to electroplate inall solutions following Step 10. In addition to the obvious problem ofwastefully plating out substantial amounts of metal on the rack, themore serious effect of rack plating is that it wastefully consumeselectrical current thereby robbing the parts to be plated of theelectrical energy needed for adequate build-up of the required platecomposite. Plated deposits on the racks themselves introduce a shieldingeficct in respect to the articles supported by them, reducing thecurrent density on the parts to be plated during the electroplatingprocess, thus requiring greater total plating time for a desired platethickness. Even more seriously, the effect of rack plating and theresulting reduction of plating current density on the parts results inmisplating or incomplete coverage, especially in the chromium platingbath where below a given current density satisfactory plating cannot beaccomplished.

Attempts to obviate the ditficulties have usually been of three generaltypes. In the first, parts to be plated are mounted on a different rack,or are re-racked, following Step 9 or 10 in the foregoing outline. Thisresults in additional manpower requirements and an increase in laborcost. Another procedure employed has been to eliminate all racking ofthe articles prior to Step 11 since the operations involved in thechemical plating portion of the overall cycle, namely Steps l-lO,require no external electrical current to be supplied to the parts andthey can be immersed in the various steps in bulk, i.e. in baskets,plating barrels or the like. Thereafter, the parts are mounted on asuitable plating rack for processing through the electroplating steps.The obvious drawback of this method arises from increased contaminationof the various solutions by carry-over and drag-in during transfer ofthe parts in bulk from one solution to the next. Furthermore there issubstantial opportunity for damaging the parts through rubbing andscratching against each other since they are simply jumble-packed in thebasket or barrel employed in transporting them through the severalprocessing steps. This is especially prevalent if the parts are largeand heavy. Finally, attempts have been made to reduce the disadvantagesof rack plating while still utilizing the racks to afford a continuousprocessing cycle without re-racking and partially successful effortshave resulted from so controlling the nature and composition of thematerials used in Steps 4 through 9 as to provide sutlicient drivingforce to effect chemical plating of the parts but not of the plasticsheathed or coated portion of the racks. There is, however, greatpractical difficulty in commercial use of this method because it narrowsthe latitude of the operating conditions, i.e. concentration,temperature and permissible processing time in the various steps, andaccordingly there is a constant struggle between incomplete partcoverage and partial rack coverage.

It is therefore the principal object of this invention to provide amethod of so treating the racks that the plastic sheathed portion doesnot become plated, yet plating of the articles themselves is in no Wayadversely affected. A further object of the invention is to provide amethod which may be incorporated directly in the normal plating cycleand be compatible with such cycle. Thus the invention here disclosedprovides a method that permits a continuous plating operation to beemployed for both chemical and electrodeposition portions of the overallplating cycle without reracking of the articles at any time throughoutthe cycle. Such continuous uninterpreted sequence of process operations,from the initial step of mounting the parts on the rack to the finalstep of unloading the finished plated parts from the rack, is especiallyimportant in high speed automated plating operations.

It has been discovered that inclusion of hexavalent chromium ions, -i.e.Cr, in the superficial layer of plastic sheathing on the rack results insubstantial reduction or complete elimination of rack plating. While theinclusion of chromium ions in the plastic surface may be accomplished innumerous ways, this can be simply accomplished by soaking the racks inan acidic aqueous solution rich in chromic oxide, thereby diifusinghexavalent chromium from solution into the plastic sheathing of therack. Such an operation can be readily incorporated directly in thesequence of steps through which a rack is advanced by the conventionalconveyor in traversing a complete cycle of operations in the platingsystem. Notwitstanding the fact that the plastic sheathing of the racksis subjected in each cycle to the reducing action of the chrome-killsolution at Step 4 in the cycle described above, sufiicient selectivityexists between the plastic parts themselves and the plastic sheathingsuch that the chromium ion level in the sheathing can be and ismaintained high enough in accordance with the procedure here outlined toprevent rack plating while allowing normal plating of the partssupported on the racks to proceed. The following examples will serve toillustrate the nature of this invention.

EXAMPLE 1 Nylon parts are mounted on a vinyl plastic coated rack whichhas been previously soaked for 5 minutes in a solution containing 6pounds of chromic acid per gallon equivalent to 45% by weight at 140 F.using a plating cycle as hereinabove described, with the exception ofeliminating Step 3 and inserting the antiplate treatment of the racks,Step A, ahead of Step 1. Parts are successfully plated by firstdepositing an initial layer of copper or nickel in the electroless orchemical plating portion of the cycle, followed by nickel and/orchromium plating in the electroplating portion of the cycle, all withoutre-racking.

EXAMPLE 2 ABS parts are mounted on a vinyl plastic coated rack andplated in a cycle similar to the foregoing, except that Steps 1 and 2are eliminated so that the sequence of steps then becomes Step A, 3,etc. In this case, Step A comprises immersing the rack and supportedarticles in an aqueous solution containing 4 pounds of chromic acid pergallon (approximately 33% by weight) for a period of 30 seconds at 120F. No plating of the plastic coating of the racks occurs, while completecoverage of the ABS parts is accomplished.

EXAMPLE 3 Polypropylene parts are mounted on a vinyl plastic coated rackand plated through the previously described conventional plating cyclecommencing with Steps 1 and 2 but the procedure is then modified toinsert Step A directly ahead of Step 3. Step A involves the same type ofanti-plate composition as Step A but in this particular example thesolution contains 1.5 pounds of chromic acid per gallon (approximately16% by weight), and the plastic coated rack and parts are immersed in itfor 2 minutes at 110 F. Following this, the process is resumed at Step 3and continues with the balance of the procedure as outlined. Thisresults in complete electroplate coverage of the polypropylene partswith no plating on the plastic coated rack.

EXAMPLE 4 The plastic coating employed commercially is commonly a PVC(polyvinyl chloride) base resin material which can be applied by dippingthe racks in a solution of such material and either evaporating asolvent carrier if such is used or otherwise curing the polymer materialto form an adherent film or coating on the rack. However, the method ofthe present invention is likewise applicable to plating racks coatedwith polypropylene sheathing, provided the articles to be plated are notalso polypropylene, and when treated in accordance with the invention,such sheathing will not acquire a metal plate in traversing through themetallizing operations described. For example, a polypropylene sheathedrack used in supporting nylon parts following the procedure of Example1, results in complete plating of the parts with no plating of the rack.

' EXAMPLE 5 Similarly ABS parts are mounted on polypropylene sheathedracks and processed as in Example 2. Again complete metal coverage ofthe parts is obtained with no deposit of metal on the racks.

EXAMPLE 6 Polystyrene parts supported on vinyl plastic coated racks,following the procedure in Example 3, also give complete articlecoverage without encountering rack plating.

EXAMPLE 7 Polysulfone parts supported on vinyl plastic coated racks areprocessed in the same manner as in Example 3 above, using the samechromic acid concentration, time and temperature conditions as in theaforesaid example. Complete plating of the parts is obtained with norack plating.

In each of the foregoing examples, the treatment produces a distinctivegreen-brown coloration of the rack sheathing due apparently toadsorption of hexavalent chromium, and it is readily observed that thefreedom from rack plating is strongly dependent on the degree ofcoloration.

EXAMPLE 8 ABS parts supported on plastic coated racks are processed inaccordance with Example 2 above, but in this case Step A comprisesimmersing the rack and supported articles in a mixed chromic-sulfuricacid solution having a high Baum in which the hexavalent chromium ioncontent is only about 1.4% by weight while the sulfuric acid is aboutEven when using an excessive processing time in Step A of severalminutes and a temperature of around F., rack plating readily occurs. Itis also noted that the characteristic deep green-brown coloration of theplastic coating, which results in each of the previous examples afterprocessing in Step A or Step A, is not obtained in this case.

EXAMPLE 9 A further modification of the procedure outlined in Example 8,in which the hexavalent chromium ion concentration in Step A is raisedto 12-14% by weight, while the sulfate (sulfuric acid) concentration isreduced to 40-50%, produces a noticable decrease in rack plating and mayin some cases provide acceptable operating conditions where the platingactivator employed subsequently is reasonably selective, and/or wherethe plastic material of the parts being plated is of the relatively moreeasily platable type. Here again the characteristic green-browncoloration of the plastic rack coating is observable, thoughsubstantially lighter in color than that obtained in Examples 1 through6.

It is apparent from the foregoing illustrative examples and from otherresearch conducted in connection with this invention that the hexavalentchromium ion must be present in the anti-rack plating solution at aminimum concentration of about (by weight), but at this level, it willgenerally be necessary to employ longer treatment times and highersolution temperatures. Optimally it is in the range of 16%, as seen fromExample 3, but may go as high as (Example 1) or even to saturation(about 52% by weight in straight aqueous solution). The straight aqueouschromic acid solution gives best results and is preferred in mostinstances, but sulfate ion can be present provided its concentration isnot in excess of by weight and preferably this should be kept below 40%,as seen by Example 9.

What is claimed is:

1. In the process of plating non-conductive articles with a metal in acontinuous plating cycle wherein the articles are suspended from platingracks and advanced without re-racking through a complete operating cyclecomprising first chemically plating an initial metal deposit on thearticles, including the steps of subjecting the racked articles toorganic solvent and/or acid etch pretreatment, sensitizing, activatingand electroless deposition of metal, and then electrodepositing afurther metal late on the chemically plated articles to build up a finaldesired total thickness and type of metal finish thereon, and whereinsaid racks are metal but all portions subjected to immersion in theplating solutions during the cycle of plating operations are encased ina plastic sheath except the rack tips from which the articles aresuspended: the improvement to reduce the deposit of plating metal onsaid racks which comprises subjecting at least said sheathed portionthereof to an anti-plating treatment comprising immersing the racks inan aqueous acid solution containing hexavalet chromium ions in adequateconcentration and for sufiicient length of time to effect adsorption ofhexavalent chromium in the surface of said plastic sheath.

2. The process as defined in claim 1, wherein said rack anti-platingsolution contains hexavalent chromium ions at a concentration of atleast 10% by Weight.

3. The process of continuous plating of non-conductive articlessupported on plastic sheathed racks as defined in claim 1, wherein thearticles are formed of acrylonitrile-butadiene-styrene resin and thesheathed portions of the racks are of polyvinyl chloride orpolypropylene, which comprises the steps of racking the articles andthen immersing them while in racked condition in an aqueous solution ofchromic acid to produce adsorption of hexavalent chromium in thesheathed portions of the racks equivalent to that resulting fromimmersing them in a solution containing four pounds of chromic acid pergallon for a period of 30 seconds at F.; and thereafter advancing saidracked articles directly to the usual acid etch pretreatment step andproceeding with the remainder of said chemical and electroplatingoperations.

4. The process of continuous plating of non-conductive articlessupported on plastic sheathed racks as defined in claim 1, wherein thearticles are formed of nylon and the sheathed portions of the racks arepolyvinyl chloride or polypropylene, which comprises first immersing theracks in an aqueous solution of chromic acid to produce adsorption ofhexavalent chromium in the sheathed portions equivalent to thatresulting from immersing them in a solution containing six pounds ofchromic acid per gallon for a period of 5 minutes at F.; thereafterracking the articles on the pretreated racks and progressing themsequentially in racked condition through the said plating operations butomitting said acid etch pretreatment step.

5. The process of continuous plating of non-conductive articlessupported on plastic sheathed plating racks as defined in claim 1,wherein the articles are formed of polypropylene or polystyrene and thesheathed portions of the racks are polyvinyl chloride, which comprisesthe steps of racking the articles and then immersing them in such rackedcondition, after said organic solvent but before said acid etchpretreatment steps, in an aqueous solution of chromic acid to produceadsorption of hexavalent chromium in the sheathed portions of the racksequivalent to that resulting from immersing them in a a solutioncontaining two pounds of chromic acid per gallon for a period of twominutes at 100 F.

References Cited UNITED STATES PATENTS 3,370,974 2/1968 Hepfer 117-473,142,581 7/1964 Leland 204-30 3,471,376 10/1969 Sambestre et al. 20420OTHER REFERENCES Plating on Plastics by H. Narcus Plating, August 1968,pp. 816-820.

JOHN H. MACK, Primary Examiner T. TUFARIELLO, Assistant Examiner US. Cl.X.R.

ll7-47A, 138.8R; 20430

